Coated ceramic article

ABSTRACT

A ceramic article for use at high temperatures and in corrosive environments comprises a ceramic substrate on which is deposited a substantially non-porous coating of one or more precious metals or alloys thereof.

This invention relates to a coated refractory ceramic article.

BACKGROUND OF THE INVENTION

Refractory ceramics are used in a wide variety of industries and in awide range of applications, often where high temperature, corrosiveenvironments are involved, such as in the manufacture of glassmaterials, products and components, where ceramic thermocoupleprotection sheaths, bubbler tubes, orifice rings, stirrers and otherprocess equipment are employed.

For example, the measurement of the temperature of hot molten glass isperformed by a thermocouple shielded in a protecting sheath. Suchmeasurement presents a variety of problems, associated with thetemperature involved, the high viscosity and abrasiveness of the moltenglass, the chemical reactivity of the glass and the combustionatmosphere in which it is heated. The function of the protecting sheathis to retain the thermocouple in an environment where it is shieldedfrom mechanical and chemical damage. Commonly, the protecting sheath isof alumina. However, while such sheaths have high temperature capabilitythey are relatively brittle. Additionally they suffer attack by moltenglass which is often .sufficiently severe to cause rapid failure andtotal loss of protection for the thermocouple.

Platinum group metal (PGM) and platinum group metal alloys have beenused as alternative shielding materials, as too have zirconia grainstabilized (ZGS) versions of the alloys. Maximum structural integrity ofthese sheaths requires metal thicknesses of 0.5-0.8 mm, so the sheathsare very expensive. Internal support by ceramic tubes has allowed thisthickness to be reduced to its present commonly used level ofapproximately 0.3 mm, but results in sheaths which require mechanicaldesign compromises and which are still too intrinsically expensive.

Apparatus fabricated with a metallic substrate that is coated or cladwith platinum group metal provide protection and enable the service lifeto be increased, but application temperatures are limited by substratemelting point. For example, the use of Ni alloy substrates limitsworking temperature to below 1300° C. and in most cases below 1200° C.The use of refractory metals can extend working temperatures to as highas 1600° C. but the penalty paid for this is the need to protect allsurfaces which might ever see temperatures greater than 400°-600° C.Ceramic substrates offer an alternative vehicle for applicationparticularly in this higher temperature range.

Platinum group metal-clad ceramics have traditionally filled thistemperature niche. However, the air gap which is present between themetal cladding and the ceramic reduces the responsiveness of theenclosed thermocouple to temperature changes and the sheaths producedsuffer from poor resistance to thermal and mechanical shock. Cladceramics are two-piece structures and the components have the attributesof metals for the cladding, and ceramics for the substrate.

Traditionally, ceramics directly coated with platinum group metals havefailed to produce economically articles with durability suitable for usein high temperature and corrosive environments. Traditional coatingprocesses suffer from stress build-up during deposition and generallyprovide a limitation for the coating thickness via a particularapplication route. Such thicknesses are generally insufficient toprovide the protection required. Where a coating process has beencapable of producing coatings of a satisfactory thickness, otherproblems have traditionally been manifest,: eg with adhesion, and/ormechanical strength, and/or integrity, and/or porosity.

GB 1242996 (Coming Glass) discloses a method of applying a platinumcoating onto ceramics by plasma spraying. It is said that flame sprayingis not capable of producing adherent, non-porous coatings. We are notaware that coated ceramics produced according to the process of GB1242996, or by any other process, were ever successfully commercialized.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a ceramic articlesuitable for use in any application where high temperatures and/orcorrosive environments are present, which article overcomes at leastsome of the problems of the prior art.

Accordingly, this invention provides a ceramic article for use at hightemperatures and in corrosive environments comprising a refractoryceramic substrate on which is deposited a coating of one or moreprecious metals or alloys thereof said coating having a thickness offrom 50 to 350 microns and being substantially non-porous.

The invention further provides a method of making a ceramic article foruse at high temperatures and in corrosive environments, comprisingapplying to a refractory ceramic substrate by flame spraying a coatingof one or more precious metals or alloys thereof in a thickness of from50 to 350 microns, and making the coating substantially non-porous.

The substrate may be any suitable oxide or non-oxide engineering orrefractory ceramic material such as alumina, alumina silicate,zircon-mullite, zirconia, mullite, silica or titania. Preferably, thesubstrate is an alumina- or zirconia-based material, or a mullite-basedmaterial. Most importantly, the ceramic and the coating system must becompatible in their thermal expansion coefficients in order to avoidundesirable stresses in the article in use.

The substantially non-porous coating is chosen from one or more of theprecious metals. Preferably the coating is of platinum or an alloy ofplatinum e.g. platinum with 5% gold, or platinum with 10% iridium,platinum with 5% ruthenium, or platinum with 10% rhodium, or platinumwith up to 1% zirconium. Lower or higher percentages of the alloyingmetals may also be used, including compositions traditionally difficultto manufacture in forms suitable for cladding. Other platinum groupmetals may also be employed particularly palladium and alloys based onpalladium.

The coating is deposited by flame spraying, for example by high velocityoxy fuel gas spraying. In a preferred flame spraying process, the sourceof precious metal is in wire form, rather than the powder required bythe other techniques. Suitable equipment is commercially available. Theexternal surface of the coating is made substantially non-porous bytreatment of the coating after deposition comprising, for example, shotpeening, flame glazing or mechanical burnishing. The thickness of thecoating is typically 50-350 μm, such as 100-350 μm, and is preferably atleast 125 μm.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The article of the present invention benefits from a range ofattributes. It has good resistance towards the diffusion of oxidantsthrough to the substrate provided by a high aspect ratio microstructureof the coating, and requires considerably less precious metal than theprior art articles. The intimate contact achieved between the ceramicsubstrate and the metal coating eliminates any thermocouple responselimitations due to air gap considerations. The high quality of thestructure and its inherent thermal and electrical properties allowsplatinum group metal and alloy coatings to act as electromagneticshields for the thermocouples, and hence improve their signal qualitywith reduced noise levels. The composite structure achieved by thecreation of sound interfacial characteristics, related to the substratesurface conditions, and carefully controlled coating depositionparameters, allows the metal coating to lend metal attributes to theceramic and vice versa, and results in a structural material superior tothat of the prior-art clad ceramic. This allows use of the componentwhere thermal and mechanical stresses might previously have been toogreat. The achievement of the sound interface may be brought about bypreliminary, chemical, mechanical or thermal preparation of thesubstrate. Such treatments might include machining, grit blasting,degreasing, or acid etching.

The process employed in this application produces a low stress deposit,which can be built up to structural thicknesses if required. Complexshaped structures which cannot readily be clad, may be protected. Thethickness of the coating can be varied substantially to ensure thecorrect level of protection is available in different areas of thecomponent. Also, once the correct interfacial characteristics have beenachieved, further depositions of the same or different PGMs and alloyscan be put down on local regions.

Oxides or inert particulates may be used to pin grain boundaries in thecoatings of the present invention. At high temperatures over longperiods of time, platinum group metal based alloys, and particularlypure metals, can exhibit extensive structural change and grain growth.These characteristics can lead to in-service failure. Pinning grainboundaries can offset or delay such microstructural changes. The inertparticulates, such as zirconia in ZGS alloys, may be included directlyin the coating during deposition, or be generated by post-depositiontreatment, depending on the requirements with regard to composition,size and distribution. Conventionally, claddings of ZGS alloys have beenprepared, but these have required joining to create complex shapes.Joints can in any circumstance be potentially weak, thus the eliminationof the need for joining by the use of coating technology is a:significant advance.

If desired an additional coating may be added between the substrate andthe precious metal coating, either individually or comprising a mixtureof one or more ceramics and one or more precious metals or alloysthereof. The additional coating may be formed by thermal spraying, forexample by flame or vacuum- or air- plasma spraying, or by thedeposition of one or more layers. A plurality of layers may be employedto form a step-wise gradation of the ceramic to metal ratio.Conventionally fabricated PGM product, eg sheet, strip, tube, wire canbe joined to a coated component by traditional technique. Suchtechniques could include arc welding, hammer welding, laser- or ion-beambonding, and spray bonding. This enables the best features of bothprocesses to be utilized.

The invention will now be described by Example. Example 13 describesindustrial service trials of the coated articles.

EXAMPLES Examples 1-4

In each case, a sheath was coated by directly flame spraying a coatingof platinum onto the substrate. A commercial flame spraying apparatususing propylene and oxygen to form the flame, and compressed air toatomize the platinum, was used. Platinum wire was fed from a reel to theflame. An after-treatment of shot-peening was applied. Testing of thecoated samples was by partial immersion in flint glass held at a nominaltemperature of 1350° C. for 100 hours.

    ______________________________________                                        Sample Coating  Thickness Substrate                                                                             Result                                      ______________________________________                                        E1     Pt       100 μm Mullite Coating pin-holed                                                             above glass,                                                                  otherwise OK.                               E2     Pt       250 μm Mullite Performed well,                                                               coating                                                                       protective.                                 E3     Pt       100 μm Impervious                                                                            Pin-holing of                                                         aluminous                                                                             coating above                                                         porcelain                                                                             glass level.                                E4     Pt       250 μm Impervious                                                                            Performed well.                                                       aluminous                                                                             Coating                                                               porcelain                                                                             protective.                                 ______________________________________                                    

Examples 5 and 6

In each case, a sheath was coated by directly flame spraying a coatingof platinum onto the substrate. An after-treatment of shot-peening wasapplied.

Testing of directly deposited, coated samples was by partial immersionin amber glass held at a nominal temperature of 1350° C.

    __________________________________________________________________________    Sample                                                                             Coating                                                                             Thickness                                                                            Substrate                                                                           Test  Result                                          __________________________________________________________________________    E5   Pt    175 μm                                                                            Mullite                                                                             200 hours                                                                           Slight grain                                                                  growth.                                                                       Coating protective.                             E6   Pt    400 μm                                                                            Mullite                                                                             400 hours                                                                           Grain growth.                                                                 Coating protective                                                            and supported                                                                 cracked substrate.                              __________________________________________________________________________

Examples 7-9

In each case, a sheath of alumina, Development Sample Number MX3supplied by Morgan Matroc Limited, was coated by directly flame sprayingplatinum onto the proprietary alumina. An after-treatment ofshot-peening was applied. Testing of the coated samples was by partialimmersion in flint glass held at a nominal temperature of 1250° C. for300 hours.

    ______________________________________                                        Sample Coating   Thickness  Result                                            ______________________________________                                        E7     Pt        186 μm  Coating fully protective.                         E8     Pt        165 μm  Coating fully protective.                         E9     Pt        178 μm  Coating fully protective.                         ______________________________________                                    

Example 10

The coating was directly flame sprayed onto a mullite sheath, and anafter-treatment of shot-peening was applied. The sample was tested bypartial immersion in amber glass at 1250° C. for 300 hours.

    ______________________________________                                        Sample Coating    Thickness  Result                                           ______________________________________                                        E10    10% Ir/Pt  200 μm  Coating discoloured but                                                       protective.                                      ______________________________________                                    

Example 11

The coating was directly flame sprayed onto a sheath of mullite. Thesample was ;thermally treated at 1200° C. for one hour in air, and anafter-treatment:of shot-peening was applied. Testing was at 1250° C. for300 hours in amber glass.

    ______________________________________                                        Sample Coating   Thickness  Result                                            ______________________________________                                        E11    Pt-Zr     200 μm  Substrate protected.                                                          Grain growth restricted.                          ______________________________________                                    

Example 12

The coating was directly flame sprayed onto an aluminous porcelainsheath, and an after-treatment of shot-peening was applied. The samplewas tested by partial immersion in flint glass held at a nominaltemperature of 1430° C. for 40 hours.

    ______________________________________                                        Sample Coating  Thickness Result                                              ______________________________________                                        E12    Pt       150 μm Pt coating provided protection.                     ______________________________________                                    

Example 13

Service Trials

Case 1

A coating of 10% Rh/Pt was flame sprayed directly onto a mullite sheath,and locally thickened at the glassline to 250 μm from 200 μm below theglass and 175 μm above the glass. An after-treatment of shot-peening wasapplied.

The coated article was tested in a commercial forehearth at 1140°-1220°C. in amber, flint and green glasses, at various times. The totalservice life is greater than eight months, the coated article stillbeing on test at that stage.

Case 2

A twin layer of Pt on grain stabilised platinum was flame sprayeddirectly onto a mullite sheath, and shot-peened. The coating thicknesswas between 225 μm and 275 μm. On testing in a forehearth used formaking amber glass bottles, service life was 1000 hours. The article wasthen removed for examination. The substrate was essentially protected.

Case 3

A silica sphere, thermally treated at 1400° C. for ca 1 hour, wasdirectly flame sprayed with a 250 μm thick Pt coating. Anafter-treatment of shot-peening was applied.

The coated article was tested at 1430° C. for 1.75 hours. Thetemperature was cycled to an estimated 1200° C. and back once everyminute, ie approximately 100 cycles.

The coating performed well but was cracking due to major failure of thesubstrate.

Case 4

An alumina-silicate sphere, thermally treated at 1400° C. for ca 1 hour,was directly flame sprayed with a nominal 250μm thick Pt coating. Anafter-treatment of shot-peening was applied.

The coated article was tested at 1340° C. (maximum) for more than 72hours, and repeatedly cycled between maximum temperature and anestimated 1000° C. each minute.

The coated article was then cooled to room temperature and re-heated tooperating temperature.

Several overnight soaks at a temperature of 1300° C. or 700° C. werealso included. The coating performed extremely well, with the substrateessentially protected.

Case 5

Coating/Clad Weld

A 300 μm Pt coating was flame sprayed directly onto a mullite tube. Atube of Pt, 0.5 mm thick, was slipped over the sample, and welded usinga standard tungsten inert welding (TIG) processor. The joint was intact.

The integrity was further assessed by raising the temperature to 1300°C. for one hour and then cooling to room temperature. No evidence ofdamage to the coating or joint was observed.

We claim:
 1. A ceramic article for use at high temperatures and incorrosive environments, comprising a refractory ceramic substrate onwhich is flamed sprayed a coating of at least one precious metal oralloys thereof, said coating having a thickness of from 50 to 350 μm andbeing non-porous.
 2. An article as claimed in claim 1, wherein thesubstrate is selected from the group consisting of alumina and zirconia.3. An article as claimed in claim 1, wherein the substrate is mullite.4. An article as claimed in claim 1, wherein the coating is selectedfrom the group consisting of platinum and platinum alloys.
 5. An articleas claimed in claim 4, wherein the coating is selected from the groupconsisting of at least one of platinum, zirconia grain stabilizedplatinum, and zirconia grain stabilized platinum alloys.
 6. An articleas claimed in claim 4, wherein the alloy of platinum comprises platinumwith 5% gold, or platinum with 5% ruthenium or platinum with 10%rhodium.
 7. An article as claimed in claim 1, wherein the thickness ofthe coating is at least 125 μm.
 8. An article as claimed in claim 1,wherein an additional coating comprising at least one ceramics materialand at least one precious metal or alloys thereof, is positioned betweenthe substrate and the precious metals coating.
 9. An article as claimedin claim 1, wherein the flame sprayed coating, after application, has asurface which is densified by shot peening.
 10. An article as claimed inclaim 1, wherein the flame sprayed coating, after application, has asurface which is densified by flame glazing.
 11. An article as claimedin claim 1, wherein the flame sprayed coating, after application, has asurface which is densified by mechanical burnishing.
 12. A ceramicarticle for use at high temperatures and in corrosive environments,comprising a refractory ceramic substrate containing a flame sprayedcoating of at least one precious metal or alloys thereof, said coatinghaving a thickness of from 50 to 350 μm, said coating having anon-porous surface formed by mechanical densification of said coatingafter application thereof.